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>Why does this strange 'x io error' occur when I
fork() in my GTK+ app? [GTK 2.x]</TITLE
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NAME="AEN505"
>Why does this strange 'x io error' occur when I
<TT
CLASS="LITERAL"
>fork()</TT
> in my GTK+ app? <I
CLASS="EMPHASIS"
>[GTK 2.x]</I
></A
></H1
><P
>This is not really a GTK+ problem, and the problem is
not related to <TT
CLASS="LITERAL"
>fork()</TT
> either. If the 'x io
error' occurs then you probably use the <TT
CLASS="LITERAL"
>exit()</TT
> function
in order to exit from the child process.</P
><P
>When GDK opens an X display, it creates a socket file
descriptor. When you use the <TT
CLASS="LITERAL"
>exit()</TT
>
function, you implicitly close all the open file descriptors,
and the underlying X library really doesn't like this.</P
><P
>The right function to use here is
<TT
CLASS="LITERAL"
>_exit()</TT
>.</P
><P
>Erik Mouw contributed the following code example to
illustrate handling fork() and exit().</P
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CLASS="PROGRAMLISTING"
>/*-------------------------------------------------------------------------
 * Filename:      gtk-fork.c
 * Version:       0.99.2
 * Copyright:     Copyright (C) 1999, Erik Mouw
 * Author:        Erik Mouw &#60;J.A.K.Mouw@its.tudelft.nl&#62;
 * Description:   GTK+ fork example
 * Created at:    Thu Sep 23 21:37:55 1999
 * Modified by:   Erik Mouw &#60;J.A.K.Mouw@its.tudelft.nl&#62;
 * Modified at:   Thu Sep 23 22:39:39 1999
 * Modified by:   Tony Gale &#60;gale@gtk.org&#62;
 * Modified at:   Wed Jan 14 12:38:00 2004
 *-----------------------------------------------------------------------*/
/*
 * Compile with:
 *
 * cc -o gtk-fork gtk-fork.c `pkg-config gtk+-2.0 --cflags --libs`
 *
 */

#include &#60;stdio.h&#62;
#include &#60;stdlib.h&#62;
#include &#60;signal.h&#62;
#include &#60;sys/types.h&#62;
#include &#60;sys/wait.h&#62;
#include &#60;unistd.h&#62;
#include &#60;gtk/gtk.h&#62;

void sigchld_handler(int num)
{
  sigset_t set, oldset;
  pid_t pid;
  int status, exitstatus;

  /* block other incoming SIGCHLD signals */
  sigemptyset(&#38;set);
  sigaddset(&#38;set, SIGCHLD);
  sigprocmask(SIG_BLOCK, &#38;set, &#38;oldset);

  /* wait for child */
  while((pid = waitpid((pid_t)-1, &#38;status, WNOHANG)) &#62; 0)
    {
      if(WIFEXITED(status))
	{
	  exitstatus = WEXITSTATUS(status);

	  fprintf(stderr, 
		  "Parent: child exited, pid = %d, exit status = %d\n", 
		  (int)pid, exitstatus);
	}
      else if(WIFSIGNALED(status))
	{
	  exitstatus = WTERMSIG(status);

	  fprintf(stderr,
		  "Parent: child terminated by signal %d, pid = %d\n",
		  exitstatus, (int)pid);
	}
      else if(WIFSTOPPED(status))
	{
	  exitstatus = WSTOPSIG(status);

	  fprintf(stderr,
		  "Parent: child stopped by signal %d, pid = %d\n",
		  exitstatus, (int)pid);
	}
      else
	{
	  fprintf(stderr,
		  "Parent: child exited magically, pid = %d\n",
		  (int)pid);
	}
    }

  /* re-install the signal handler (some systems need this) */
  signal(SIGCHLD, sigchld_handler);
  
  /* and unblock it */
  sigemptyset(&#38;set);
  sigaddset(&#38;set, SIGCHLD);
  sigprocmask(SIG_UNBLOCK, &#38;set, &#38;oldset);
}

gint delete_event(GtkWidget *widget, GdkEvent *event, gpointer data)
{
  return(FALSE);
}

void destroy(GtkWidget *widget, gpointer data)
{
  gtk_main_quit();
}

void fork_me(GtkWidget *widget, gpointer data)
{
  pid_t pid;

  pid = fork();

  if(pid == -1)
    {
      /* ouch, fork() failed */
      perror("fork");
      exit(-1);
    }
  else if(pid == 0)
    {
      /* child */
      fprintf(stderr, "Child: pid = %d\n", (int)getpid());

      execlp("ls", "ls", "-CF", "/", NULL);
      
      /* if exec() returns, there is something wrong */
      perror("execlp");

      /* exit child. note the use of _exit() instead of exit() */
      _exit(-1);
    }
  else
    {
      /* parent */
      fprintf(stderr, "Parent: forked a child with pid = %d\n", (int)pid);
    }
}

int main(int argc, char *argv[])
{
  GtkWidget *window;
  GtkWidget *button;

  gtk_init(&#38;argc, &#38;argv);

  /* the basic stuff: make a window and set callbacks for destroy and
   * delete events 
   */
  window = gtk_window_new(GTK_WINDOW_TOPLEVEL);

  g_signal_connect(G_OBJECT (window), "delete_event",
		     G_CALLBACK(delete_event), NULL);
          
  g_signal_connect(G_OBJECT (window), "destroy",
		     G_CALLBACK(destroy), NULL);

#if (GTK_MAJOR_VERSION == 1) &#38;&#38; (GTK_MINOR_VERSION == 0)
  gtk_container_border_width(GTK_CONTAINER (window), 10);
#else  
  gtk_container_set_border_width(GTK_CONTAINER (window), 10);
#endif

  /* add a button to do something useful */
  button = gtk_button_new_with_label("Fork me!");
          
  g_signal_connect(G_OBJECT (button), "clicked",
		     G_CALLBACK(fork_me), NULL);

  gtk_container_add(GTK_CONTAINER(window), button);
          
  /* show everything */
  gtk_widget_show (button);
  gtk_widget_show (window);


  /* install a signal handler for SIGCHLD signals */
  signal(SIGCHLD, sigchld_handler);

  
  /* main loop */
  gtk_main ();

  exit(0);         
}</PRE
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